Numerous timepieces provided to indicate the phases of the moon are already known, but the problem of indicating the phases of the earth entails an additional technical difficulty.
A man skilled in the art knows that the rhythm with which the phases of the moon are connected and the rhythm with which the phases of the earth are connected are identical and further, that the phases of the moon and the phases of the earth are exactly in opposition. At first glance, therefore, producing a watch indicating the phases of the earth, with the aid of gearwheels conventionally used to indicate the phases of the moon, should present no difficulty. However, unlike the moon which always presents the same face to the earth, the latter, as is well known, turns upon its axis at the rhythm of one rotation every 24 hours. An astronaut situated on the moon thus sees the topography of the continents present in the visible part of the earth gradually change. This rotating effect, which combines with the series of phases of the earth, has no equivalent in the moon.